The invention relates to a process for producing carbon fillers having covalently bonded amino groups.
Carbon fillers have been used as a filler for some time, especially in polymer molding compositions. Examples of such carbon fillers are (conductive) black, graphite, carbon nanotubes or graphenes. It is also possible to use activated carbon or carbon fibers. Use is not restricted to customary filler applications, and it is also possible to conceive of applications, for example in the electronics and storage media sector, such as electrical conductors and transistors, electrode materials, storage media, etc.
According to the prior art, carbon nanotubes (CNTs) are understood to mean principally cylindrical carbon tubes with a diameter of about 3 to 100 nm and a length of several times the diameter. These tubes consist of one or more layers of ordered carbon atoms and have a core of different morphology. They are also referred to, for example, as “carbon fibrils” or “hollow carbon fibers” and are available in different forms (for example bamboo or onion type).
Typical structures of these carbon nanotubes are those of the cylinder type. The cylindrical structures are distinguished between the single-wall monocarbon nanotubes (SWCNTs)), double-wail carbon nanotubes (DWCNTs) and multiwall cylindrical carbon nanotubes (MWCNTs)). Common processes for production thereof are, for example, arc discharge processes, laser ablation, chemical deposition from the vapor phase (CVD process) and catalytic chemical deposition from the vapor phase (CCVD process).
Carbon nanotubes are light, have high tensile strength and conduct electrical current. They have been used to date particularly as additives for polymers.
The advantageous properties of the CNTs are, however, impaired by a series of disadvantages. CNTs have a significant tendency to agglomerate and have poor solubility in polar or nonpolar solvents. One means of remedying this disadvantage consists in the application of functional groups, for example amino groups, to the outer surface of the CNTs.
There is already extensive literature regarding functionalization of CNTs. The production of CNTs comprising amino groups is also already known (e.g. N. Karousis, N. Tagmatarchis, D. Tasis, Chem. Rev. 2010, 110, pages 5366 to 5397). Usually, however, the CNT has to be pretreated before the application of amino groups.
The pretreatment may be a chemical conversion which leads to a functional group, for example a carboxyl group. Only in one or more further chemical steps is this functional group converted to an amino group.
However, the pretreatment may also involve a physical measure, for example a temperature, plasma or ultrasound treatment, or a mechanical treatment by grinding the carbon compound.
Combinations of chemical and physical pretreatment are likewise possible.
The aforementioned reference gives a good overview of the different kinds of functionalization of carbon nanotubes. The introduction of amino groups is shown in FIG. 2, for example. This proceeds from acid chloride groups and involves reaction with sodium azide. Alternatively, the production may proceed via the acid amides. Direct amidation of acyl chlorides is also possible,
CN-A-101774573 describes a process for aminating carbon nanotubes, in which carbon nanotubes are first pretreated by means of heat, acids and/or ultrasound treatment, and then reacted with ammonia or ethylenediamine at a temperature of 340 to 350° C. and a pressure of 6 to 11 MPa. The pretreatment makes the process very costly and inconvenient.
U.S. Pat. No. 7,794,683 likewise describes the production of aminated carbon nanotubes, wherein carboxylic acid groups are first introduced by acid treatment with sulfuric acid and nitric acid and are then converted to acyl azides by reaction with diphenylphosphoryl azide. The further reaction leads via isocyanate groups through hydrolysis to form amino-functionalized carbon nanotubes. A disadvantage of the process is the large number of reaction steps, some of which also require expensive reagents.
Eur. J. Org. Chem. 2008, pages 2544 to 2550, describes the covalent side wall functionalization of single-wall carbon nanotubes by nucleophilic addition of lithium amides in THF. Lithium amide is prepared from n-butyllithium and propylamine in dry THF. The reaction is effected at room temperature. After the reaction, oxygen is passed through the reaction mixture, which affords carbon nanotubes substituted by groups of the —NH—CH2—CH2—CH3— structure.
WO 2005/090233 describes the reductive functionalization of carbon nanotubes. For this purpose, carbon nanotubes are introduced into liquid ammonia, into which lithium is additionally introduced as a metal. This is followed by the addition of alkyl halide or aryl halide, which results in alkylation of the external surfaces of the carbon nanotubes; see FIG. 1 and example 1. The reaction is performed while cooling with the aid of acetone/dry ice, with heating to room temperature at the end of the reaction. Aminated carbon nanotubes are not described.
The invention of the processes described above requires the chemical and/or physical pretreatment of the carbon nanotubes before any functionalization. A disadvantage of such a pretreatment is that the structure of the carbon compounds can be damaged by the pretreatment. For example, ultrasound treatment of the carbon nanotubes can result in damage, as described in WO 2005/090233 in paragraph [0009]. In the case of oxidative pretreatment, the surface of the carbon nanotubes is attacked by the oxidizing agents, which leads to defect sites on the surface.